Failures in regulating valves or gas governors can be diagnosed by detecting the occurrence of stick-slip in a sliding part. Stick-slip occurs due to the state of a piston 1201, a cylinder 1202, and a contact sliding portion 1203, as illustrated in, for example, FIG. 12. For example, this stick-slip occurs when, for example, contamination incurs into the contact sliding portion 1203. Consequently, stick-slip can be detected by monitoring the state of a measured dislocation by measuring the dislocation of the piston 1201. (See Japanese Patent 3254624.)
Here a simple explanation will be given regarding the detection of stick-slip set forth in JP '624. In this detecting technique, the dislocation of the piston 1201 is detected, a first state quantity is calculated from the detected dislocation, a second state quantity is calculated from the detected dislocation, and a relationship between the first state quantity and the second state quantity obtained from the dislocation during proper operation is compared to the relationship between the calculated first state quantity and calculated second state quantity, to detect (evaluate) the stick-slip.
For example, the average of the absolute values of first-order difference values for the dislocation may be used as the first state quantity, and the root mean square of the first-order difference values of the dislocation may be used as the second state quantity. When the dislocations of the piston 1201 are detected discreetly and the ith detected dislocation is defined as Xi, then the respective state quantities can be expressed using Equation (1) and Equation (2), below (wherein N is the number of dislocation data used for calculating the state quantities):
                    Equations        ⁢                                  ⁢        1        ⁢                                  ⁢        and        ⁢                                  ⁢        2                                                                      (                      First            ⁢                                                  ⁢            state            ⁢                                                  ⁢            quantity                    )                =                              1                          N              -              1                                ⁢                                    ∑                              i                =                1                                            N                -                1                                      ⁢                                                  ⁢                                                                          X                                      i                    +                    1                                                  -                                  X                  i                                                                                                      (        1        )                                          (                      Second            ⁢                                                  ⁢            state            ⁢                                                  ⁢            quantity                    )                =                                            1                              N                -                1                                      ⁢                                          ∑                                  i                  =                  1                                                  N                  -                  1                                            ⁢                                                          ⁢                                                (                                                            X                                              i                        +                        1                                                              -                                          X                      i                                                        )                                2                                                                        (        2        )            
The frequency distribution of the absolute values (|Xi+1−Xi|) of the first-order differences of the dislocation is as illustrated in FIG. 13A and FIG. 13B. FIG. 13A illustrates the state during proper operation, wherein the frequency of occurrence falls smoothly with increasing magnitude of the difference values. On the other hand, if stick-slip occurs, then a majority of the time will be a stationary state, and then slipping will occur occasionally. Because of this, the frequencies of the first-order difference values will have high frequencies clustered around zero, as illustrated in FIG. 13B, (corresponding to the stationary state), with relatively large values at low frequencies (corresponding to the slipping state). In the state wherein this type of stick-slip occurs, the ratio of the first state quantity (the average value of the absolute values of the first-order difference values) to the second state quantity (the root mean square of the first-order difference values) will be larger than during proper operation, making it possible to contact the stick-slip by monitoring the two state quantities.
However, in the technique set forth above, there is a problem in that in some cases there will be an incorrect evaluation that there is a state of stick-slip, due to the state of control of the moving portion (the piston).
In the technique set forth above, the detection is performed through the relationship of two state quantities calculated, from the dislocation of a moving portion, by calculating the motion that is subject to stick-slip detection, divided into a stationary state and a slipping state. This makes the determination using only the dislocation of the moving portion. Because of this, if the movement (dislocation) of the moving portion is similar to that of the stick-slip state, then the evaluation will be that there is stick-slip, even if the stick-slip is not actually occurring. This results in erroneous detection.
For example, in the control of a valve stem position using a positioner, if there is a large change in the valve stem dislocation control instruction value (a setting value or set point), then the behavior of the dislocation of the valve (the moving portion) at the time of the change of the control instruction value may be similar to that of the stick-slip state.
As illustrated in FIG. 14 (a), when control instruction values for dislocations wherein the time-series signals form a square wave by alternating two values over time, then the response of the valve stem dislocation for the regulator valve will, accordingly, be measured as the dislocation measurement values for the time-series signals as illustrated in FIG. 14 (b). The first-order difference values in this type of dislocation measurement value will be as illustrated in FIG. 14 (c). In this case, as illustrated in FIG. 14 (c), the majority of the first-order difference values will be clustered near to zero, where only the values immediately after the control instruction value has changed will be large.
This behavior is identical to, the behavior of the stick-slip phenomenon wherein there is a stationary state the majority of the time, with occasional rapid movement in the slipping state. The result is that, in the technique set forth above, there will be incorrect detection of the occurrence of stick-slip when control is performed as illustrated in FIG. 14 (a). This incorrect detection tends to occur when the operating speed of the valve is high, and is particularly problematic in small valves.
The present invention is to solve the problem such as set forth above, and the object thereof is to enable the evaluation of the stick-slip state more correctly, in accordance with the state of control.